the trial of a Minnesota man, charged with killing two teenagers who broke into his home. He says it was self-defense. Prosecutors are calling it cold-blooded murder. Gio Benitez is here. Reporter: The man waited a day to call police. When they arrived, he showed them the bodies of the two teenagers. And now, the start of a tangled trial. The 65-year-old man accused of murdering two reportedly popular teens isn't saying much. As his tangled case plays out in a Minnesota court right now. The victims, teenage cousins. Who some might say look more like homecoming king and queen than burglars. But police say 17-year-old nick Brady and 18-year-old Haley broke into Byron smith's house on Thanksgiving day 2012. And what happened next was either self-defense or cold-blooded murder. This was about anxiety a and fear. And what somebody does in their own home. Reporter: Prosecutors say smith was sitting in his basement, when he heard a window breaking upstairs. When nick Brady started walking down into the basement, smith shot him twice. Then, according to court documents, smith shot him a third time, in the face. Allegedly telling investigators, I want him dead. Minutes later, Haley Kiper walked down the steps, reportedly calling Brady's name. Smith shot her, too. Telling investigators, the last time he fired was a good, clean finishing shot. Smith told police he had been living in fear after several break-ins. He pleaded not guilty, saying he shot the teens in self-defense. But in opening statements Monday, prosecutors said smith planned the killings. Lying in wait for the two teens in his basement, with a book, two guns, energy bars and a bottle of water. A clear case of premeditated murder, they say, of two teens that weren't even armed. I think the evidence will speak for that. Reporter: This morning, nick Brady's grandparents are hoping for justice. Very few people can ever say they made no mistakes. The difference is, I'm alive to tell about mine. Nick and Haley are not. Reporter: And meanwhile, smith is reportedly a retired state department security engineer. And now, his defense plans to argue that the two teens had broken into houses before. George? Gio, thanks very much. Dan Abrams has moved over here. A lot of tangled issues in this case. And people would be surprised that defending your home isn't necessarily a winning defense. It is a defense here. And you do have more ability to use deadly force in your own home when you're trying to prevent a felony or you're trying to protect yourself. But there's going to be two, key words in the law. Necessary and reasonable. Those are going to be the two questions if this jury's going to have to grapple with. Was it necessary? And was the force reasonable? The fact that these kids had been -- broken into homes before. And his home had been broken into before. Does that trigger reasonable? What goes on in his mind is relevant. What would a reasonable person in his situation have done is going to be the key question here. So, it is going to be brought up by the defense. And they're hoping that it's going to win the day. The problem for the defense is so many of the facts here, which goes well beyond just preventing a felony or defending himself. Talking about the possibility of effectively executing this woman. That's going to be a problem for the defense in this case. The key to this case is going to be jury selection. It's going to be so important what kind of people you get on this jury. Are they the kind of people who would say, hey, you break into someone's house, you're asking for it. Versus people who are going to focus very specifically on the law in this case, which has those two key words, necessary and reasonable. Okay. Dan Abrams, thanks very much.

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